Schema Evolution publication categorizer

The authors discuss extensions to the conventional relational algebra to support both aspects of transaction time, evolution of a database's contents and evolution of a database's schema. They define a relation's schema to be the relation's temporal signature, a function mapping the relation's attribute names onto their value domains, and class, indicating the extent of support for time. They also introduce commands to change a relation, now defined as a triple consisting of a sequence of classes, a sequence of signatures, and a sequence of states. A semantic type of system is required to identity semantically incorrect expressions and to enforce consistency constraints among a relation's class, signature, and state following update.